I am of the understanding that buying books online is hardly worth anything because you don’t actually own anything. Maybe you own the digital file? I’m not sure. I don’t trust the longevity of it, though.
I’m told a lot (mainly by my Dad) that its a good idea to print those out to keep them. However, I’m talking about books that are easily 200+ pages.
I don’t know if its worth the ink or money to print them out. At that point would it be better to just buy a book?
I wouldn’t waste the paper and ink. Make sure your digital files are accessible (ie, DRM-free and in a common format) and backed up in several locations, and that’s about as much of a guarantee you can get for them.